The solution to long range space combat

A short analysis of the challenges the different weapons in realistic SF face when trying to deal damage at long ranges, and a proposed solution.

Title art.

Lasers, particle beams, missiles, railguns... the standard fare in realistic SF weaponry. All have difficulty reaching targets at extreme ranges, for different reasons. Here is a short breakdown.

Lasers

Lasers face diffraction, thermal lensing and an overall poor ratio of energy input to output.

Diffraction means that the laser's energy spreads as it travels through space. Doubling the distance leads to an eight-fold decrease in the number of watts per square meter. It can be reduced by using short wavelengths (more advanced technology and lower efficiency) and large mirrors (fragile and heavy). Thermal lensing is the result of a laser heating up and producing a beam much less focused than a perfect Gaussian beam. It forces the laser equipment to be reduced to cryogenic temperatures, greatly complicating the removal of waste heat (energy-consuming heat pumps must be used) and increasing the mass penalty (radiator area increases by the fourth power to a reduction in temperature). 

Particle beams

A depiction of a particle beam weapon from Aviation Week, 1980

Particle beams are less sensitive to temperature changes, and have very good efficiency, but suffer from requiring huge accelerators and thermal blooming.

The two downsides to particle beams are interlinked. Thermal blooming is the expansion of hot plasma, usually at a rate of several kilometers per second. To produce a relatively focused beam at the target, relativistic effects are required to slow down the time perceived by the plasma, and reduce actual expansion rate. However, even velocities of 99.99% of the speed of light or better (and the necessary accelerator length to reach them) do not produce a beam that performs better than a small infrared laser (which has a large wavelength and a small mirror). The worst part is that a faster beam is hotter, so expands even quicker.

Railguns

Navy Railgun.

Railguns are simple to handle, and the kinetic energy at the muzzle is the energy that is delivered to the target. They are relatively compact, and do not require fragile equipment. 

Railguns are limited by the ranges space combat takes place at, and the melting temperature of its components. At ranges of 10-100km, railguns can efficiently accelerate projectiles to several kilometers per second velocities and still hit targets. However, the presence of lasers can push ranges to a hundred times that, and it increases further with advancing technology. Higher velocities means that the friction between projectile and rail produces higher temperatures. 

Efficiency degrades as temperatures increase, creating a vicious cycle. Railguns are expected to be limited to velocities of 7-10km/s before rails and projectiles of any material melt. Even a centigee acceleration 10m wide warship these rounds can dodge at ranges of 316km.

Missiles

Missiles are very effective in space. They require no energy input, and can easily be designed to track, catch up and collide with targets at any distance.

However, missiles can be shot down. If they are forced to operate on a miniaturized version of spaceship engines, they will suffer efficiency or performance penalties. If it is a different propulsion system entirely, then the targets can outrun missiles in certain situations. Defending against missiles is massively easier than attacking with them, so defenders will always have a one-up against attackers. 

So what is the solution?

Looking at all the strengths and weaknesses of those weapon systems, we reach the conclusion that these are the characteristics of a perfect weapon system for long range combat:

-Very low time to target
-No loss of energy in transit
-High efficiency, tolerance for high operating temperatures
-Cannot be intercepted
-Does not give defenders a mass advantage

Lasers, particle beams, missiles... all fail to fulfil all the criteria. One weapon system, however, is the solution.

Laser beam driven sail.

Instead of effective weaponry, this technology was initially devised as an interstellar propulsion system. Travelling between stars required a system that delivered energy to a spaceship in a way that fulfilled much of the same requirements as a weapon. Low time to target is the same as high travel velocity. No loss of energy in transit is a central focus. High efficiency leads to smaller infrastructure.

Lasers were found exceedingly inefficient as a propulsion system, achieving only 6.7N of thrust per gigawatt. This lead to ludicrous proposals such as Robert Forwards's 1984 concept that required 7.2TW lasers to operate years on end. 

Particle beams were an intermediate solution, producing thrust much more efficiently, but thermal blooming meant either very little of the beam's energy reached the spaceship, or thousands of beam focusing stations had to be sent along the interstellar probe.

Particle beam propulsion concept.

Midway through this research article, published in 2000 by Geoffrey A. Landis, we obtain an interesting quote:

This would lead to a thermal beam divergence of about 2 million kilometres per light year. This could be reduced if the beam particles condense to larger particles after acceleration. To reduce the beam spread by a factor of a thousand, the number of mercury atoms per condensed droplet needs to be at least a million. This is an extremely small droplet (10^-16 grams) by macroscopic terms, and it is not unreasonable to believe that such condensation might take place in the beam. As the droplet size increases, this propulsion concept approaches that of momentum transfer by use of pellet streams, considered for interstellar propulsion by Singer (1980).  

In that paragraph, Landis discusses the theoretical and practical lower limits for beam temperature, which decreases thermal blooming and expansion of the beam significantly. An alternative in the form of beam condensation was given. The accelerated particles merge to form small droplets, which expand much more slowly. 

Singer's pellet streams are also mentioned. They were first introduced in 'Interstellar Propulsion Using a Pellet Stream for Momentum Transfer' from 1979. The idea was deemed ineffective as near-lightspeed pellets would be knocked off-course by interstellar gas, and automation was needed to bring them back into position. Such automation would have to fit onto milligram sized pellets, and survive hundreds of thousands of Gs during launch. It is not available even today. 

But back to space warfare.

We do not need near-lightspeed projectiles, nor is there interstellar gas to knock projectiles off-course. The pellet gun is perfectly suited to our requirements. It shoots tiny metal projectiles from dozens to hundreds of kilometres per second. There are design challenges, but it:

-Can cross space combat ranges in sufficient time
-Does not lose energy in transit
-Extreme efficiency is possible (99.9%+)
-Operates on coilgun or particle accelerator principles
-Difficult to detect
-Can be rapid-fired to bypass interceptors and Whipple shield defences
-Negligible ammunition mass

How does it work?

Please check out Children of a Dead Earth

Let's work out the requirements in two different situations:

Plausible future combat

Target accelerates at 0.1m/s^2

Combat range is 1000km

Target profile is 100m

Extreme future combat

Target accelerates at 10m/s^2

Combat range is 10000km

Target profile is 100m

In plausible future combat, the target takes a minimum of 32 seconds to move outside of their profile. A projectile fired with zero lead time must cross 1000km in this time to hit it. The projectile velocity must be 31km/s or better.

In extreme future combat, the target takes a minimum of 3.2 seconds to move outside of their profile. A projectile fired with zero lead time must have a 3100km/s velocity to catch the target.

The pellets can be small. A 200g pellet of tungsten is a sphere only 3.3cm in diameter and can be accelerated to 31km/s at the cost of 1MJ, and it will deliver 1MJ to the target. 

A 3100km/s pellet of 2 micrograms can be accelerated with only 10MJ of energy, and it will deliver all the 10MJ to the target. 

You can reduce the pellet's velocity, and reduce the energy required by square. The excess energy will allow you to shoot several more projectiles, covering all possible positions the target will take after firing.

To reach such velocities, an inductance coilgun paired with a temperature-resistant material is required. Tungsten and boron carbide, advanced carbon materials, allow for lower efficiencies, or alternatively, higher energies. 

A 5 tesla field can eject a 0.15mm long, 30mm wide tungsten projectile within a length of 90 meters to 32km/s. Fed by a 100MW reactor, it can shoot 100 projectiles per second, divided between several barrels. If all projectiles hit the same surface, then they will impact with the force of 24kg of TNT, per second. 

Alternatively, that same power output can fire a single projectile of 1 gram to 447km/s. It would be able to hit a target 50cm across in plausible future combat. 

The extreme future combat coilgun with a 25 tesla field would also require 90 meters to deliver 10MJ of kinetic energy to a 2mm wide, 1mm long carbon nanotube cylinder.  

 Conclusion  

The pellet gun uses reasonable amounts of energy to deliver small projectile at very high velocities. Unlike a particle beam or laser, it does not suffer any loss of energy during transit, and is very efficient.

In terms of worldbuilding, warships that use pellet guns will make space combat resemble Big Gun warfare of the first half of the 20th century. Large targets will have to accelerate quickly to dodge incoming projectiles. There is a niche for nimble, small spacecraft that can dodge projectiles at much closer ranges than lumbering battleships. Smaller, rapid fire guns will deal with these 'fighters'. Lasers play a defensive role, but missiles will be obliterated by the impact of a pellet.

Staying outside of 'combat range' won't work as it does in laser-dominant warfare, as pellets do not dissipate with distance. High energy-per-pellet 'sniper' guns can deal with spacecrafts attempting to do this.  

Muzzle ports can be as small as 5cm or less. Unlike the large lens of a laser system, they allow warships to be enclosed in thick, sloped armor and fully resemble daggers in space.